CHAPTER 13 



SELECTION AND VARIABILITY 



Selection Darwinism and Genetics The States of Variability Fitness and Flexibility 

 The Effect of Linkable The Control of Recombination Balance in Homozygotes 

 and Heterozyqotes Selection and the Reservoir of Variability 

 Change of Genetic Systems : Inertia Correlated Response: 

 Capital and Subordinate Characters 



Almost from the beginning of genetics we are bound to examine 

 and discuss variation. New forms of plants and animals are the 

 materials for the study of heredity. But what happens to these 

 forms in nature ? It is obvious that a lethal mutation extinguishes the 

 individual in which it expresses itself. Those which cannot live must 

 die. It is less obvious, but no less certain, that many mutations which 

 are not lethal make their possessor's survival doubtful, or at least 

 reduce his chance of leaving progeny. 



Drosophila subobsaira, for example, uses its power of sight in 

 courtship, and Rendel finds that flies made blind by the absence or 

 abnormality of eyes are unable to mate. The genes producing this 

 eflect are not lethal somatically; but they are lethal genetically. 

 They must therefore be selected against, or as we may say, they have 

 a negative survival value. Again this is an extreme case; but any 

 mutation which impairs the faculties of its possessor must have a 

 survival value reduced by an amount proportional to the impairment. 



Selection 



The principle of selection can now be seen in much greater 

 breadth and depth than would have been possible at the beginning 

 of this book. In the hrst place we can see it operating in different 

 stages of development and at different levels of organization. The 

 defective cell which arises by gain or loss of a chromosome is 

 eliminated as readily as the defective individual. This cell also may 

 be a spore or a fertihzed egg. In the Renner eflfect we sec the defective 

 spore removed. Also we notice that it is removed owing to a defect 



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